(1) Field of the Invention
The invention relates to high speed, low power circuits. More specifically, the invention relates to a high speed, low power analog to digital converter employing a current regenerative comparator.
(2) Related Art
Converting analog signals to digital signals is generally well-known in the art. The fastest general analog to digital (A/D) conversion method has been flash conversion in which the sample signal is compared simultaneously (in parallel) to several reference signals to generate a thermometer code that can be encoded into a digital value. Flash A/D has historically been limited in use because it requires a large area, high power, and it is difficult to build accurate, multiple voltage references. Prior art flash A/D converters are implemented with comparators built from differential amplifiers. Differential amplifiers require large DC currents in order to have high speed. Maintaining proper bias over a variable voltage supply is also a problem in conjunction with operation at low supply voltages. The prior art flash A/D converters typically created the multiple voltage references on one side of the differential pair with a single voltage reference and a resistive ladder. CMOS fabrication techniques are well developed and relatively inexpensive. Moreover, CMOS circuitry permits high speed operation. Unfortunately, high quality resistors are not available in advanced CMOS digital processes. Accordingly, manufacturing high quality A/D using digital CMOS processes has been largely impossible.
The speed at which conversions can be made is very important since the higher frequency at which the A/D conversion can be performed, the wider the bandwidths that can be handled. This becomes increasingly important in video and audio applications. Moreover, small portable devices relying on A/D conversion have become increasingly common. The high power and large area of the previous flash implementations are unsuitable for portable battery operated applications. In any event, it is desirable to reduce the space and power consumption in A/D converters.
In view of the foregoing, it would be desirable to create a high-speed, low power analog to digital converter which can be wholly fabricated using CMOS digital processes. High Speed A/Ds using CMOS digital processes would permit integration of the A/D with microprocessors which can then serve high performance applications.